Production of photographic silver halide emulsions

ABSTRACT

THE FORMATION OF FINE GRAINS IN A PHOTOGRAPHIC SILVER HALIDE EMULSION FORMED IN AN AMMONIA-FREE GELATIN MEDIUM IS PROMOTED BY CARRYING OUT ANY OR ALL OF THE SIGNIFICANT STEPS IN MANUFACTURING AND COATING SUCH EMULSIONS UNDER SUB-ATMOSPHERIC PRESSURE, PREFERABLY BELOW 100 MM. HG.

United States Patent 3,732,102 PRODUCTION OF PHOTOGRAPHIC SILVER HALIDE EMULSIONS Hendrik Alfons Borginon, Mortsel, and Frans Henri Claes, Edegem, Belgium, assignors to Gevaert-Agfa N.V., Mortsel, Belgium No Drawing. Continuation-impart of abandoned application Ser. No. 774,190, Nov. 7, 1968. This application Mar. 12, 1971, Ser. No. 123,887 Claims priority, application Great Britain, Nov. 7, 1967, 50,631/67 Int. Cl. G03c 1/02 U.S. C]. 96-94 3 Claims ABSTRACT OF THE DISCLOSURE The formation of fine grains in a photographic silver halide emulsion formed in an ammonia-free gelatin medium is promoted by carrying out any or all of the significant steps in manufacturing and coating such emulsions under sub-atmospheric pressure, preferably below 100 mm. Hg.

The present application is a continuation-in-part of the patent application No. 774,190, filed Nov. 7, 1968, which is now abandoned.

The present invention relates to the preparation of photographic emulsions and more particularly to a special technique for controlling the grain size of silver halide grains in a neutral or acid environment.

As is known to those skilled in the art that the crystal growth of light-sensitive silver halide grains can be controlled by special working conditions in the preparation of light-sensitive silver halide emulsions. Thus, it is known that the grains formed during precipitation are finer at the minimum solubility of the silver halide, when the gelatin concentration is increased to a certain limit, when the temperature is reduced, or when crystallization nuclei such as silver iodide or silver sulphide are present.

Further it is known that the growth of the silver halide grains in the physical ripening step is promoted by an excess of, e.g. soluble bromide, by the presence of a silver halide-dissolving agent such as ammonia, by increasing temperature and by an increasing rate of agitation. In contrast therewith it is possible to retard the crystal growth during ripening by gelatin which is rich in retarders and by means of chemical compounds selected therefor, e.g. imidazoles, thiazoles, thiolactic acids, nucleic acids, thiosulphate, cystine and dithiopyrimidine. Cadmium ions and other bivalent cations retard the ripening of silver halide and are used e.g. in the preparation of contrasty silver chloride and silver chloro-bromide emulsions.

The well-known dispersing action of ultra-sonic waves has been used for breaking down grain aggregates and producing an improved emulsion homogeneity and grain size stability. The ripening time of silver halide emulsions can be increased and the ripening temperatures can be raised by the action of ultra-sound, without any fogging action, which normally occurs under these conditions.

The inhibition of grain growth of silver halide grains is of major importance in cases wherein it is desired to obtain a fine grain silver halide emulsion of high resolving power and suitable contrast. Silver halide emulsion layers with fine grain silver halide are especially useful in microcopying and in the production of halograms.

The present invention is based on the experimental fact that gases dissolved in an aqueous medium comprising dispersed silver halide promote the grain growth of the silver halide. Gases which have been found to be active in that respect are oxygen, nitrogen, ammonia, carbon dioxide and helium or mixtures thereof such as air.

3,732,102 Patented May 8, 1973 According to the present invention, the formation of a silver halide emulsion containing fine silver halide grains for use in a photographic silver halide emulsion material is carried out in an ammonia-free gelatin medium and all of the significant steps in the manufacturing of the silver halide emulsion and the coating of such emulsion is in order to substantially inhibit or retard the crystal growth of the silver halide grains carried out under subatmospheric conditions.

The absence of ammonia in the formation of fine grain silver halide is necessary to prevent physical ripening in the precipitation stage, since ammonia by its complex ion formation is a well known ripening agent (ref. G. F. Duffin in the book, Photographic Emulsion Chemistry, Focal Press Limited, London-New York (1966), 59).

The definition of a neutral, acid and ammonia emulsion can be found in the same book in the chapter Emulsion Preparation.

The application of a reduced pressure to remove ammonia just after or as soon as possible after the formation of the silver halide in ammonia type emulsion in order to economize in the washing of the emulsion is known from the United Kingdom patent specification 676,745.

According to the present invention, the reducing of pressure in order to retard the grain growth of silver halide is carried out in the precipitation stage in acid or neutral medium.

The removal of dissolved gases is preferably carried out by means of a vacuum pump maintaining a pressure preferably lower than mm. Hg, e.g., of about 40 to 70 mm. Hg.

An important advantage associated with the grain growth inhibition technique of the present invention resides in the fact that the medium containing silver halide remains chemically unaffected when it is subjected to reduced pressure. This is of importance in those emulsion preparation steps wherein physical ripening has to be inhibited or suppressed, e.g., during the process of chemical ripening or during another step of emulsion preparation. This is of particular importance in the preparation of silver-halide grains wherein a part or all of the silver halide of the originally formed silver halide grains is replaced (converted) by another type of silver halide without substantially changing the grain size; such a type of silver halide emulsion is called converted emulsion.

A binding agent or protective colloid used in a par ticular preparation step of the photographic emulsion does not influence the effect of reduced pressure, which may be applied to preparation steps of silver halide emulsions that are characteristic for emulsions to be washed in precipitated form, for emulsions to be set and shredded before washing, as well as for emulsions not to be washed at all.

It is possible to reduce pressure during the whole emulsion preparation process or during specific steps of its preparation. It is to be understood, however, that the invention does not include the step of de-aerating under reduced pressure a prepared emulsion prior to coating the said prepared emulsion on a support.

According to a preferred embodiment of the present invention all the steps of the preparation of acid or neutral silver halide emulsions from the formation of the silver halide nuclei till the addition of the coating aids inclusive are carried out under reduced pressure. It has also been proved that maintaining a reduced pressure during storage before coating is advantageous, so that the crystal growth is kept as low as possible before the emulsion is applied to the support.

Confining the application, however, to reduced pressure during the steps of precipitation (formation of nuclei) and chemical ripening (the physical ripening or Ostwald ripening being omitted for fine grain emulsions), still results in a very effectively inhibition of the grain growth, since little coagulation of crystals or Ostwald growth of the largest grains at the expense of the smaller ones take place in the other steps (setting, shredding, washing, etc.).

It has been established that by means of reduced pressure crystal growth can be advantageously inhibited in photographically inert binder media, i.e. in media wherein no gelatin is present, but wherein it is only used at the time of setting, washing or coating the emulsion.

The precipitation of already formed silver halide in the presence of small amounts of binder, however (see, e.g., United Kingdom patent specification 1,008,694) and the washing of the precipitate are preferably also carried out under reduced pressure.

Further, it has been established that the application of reduced pressure during the various steps of emulsion preparation has no harmful effect on the activity of emulsion ingredients, such as chemically sensitizing agents, development-accelerating compounds, grain growth retarding agents, stabilizing agents, spectrally sensitizing agents, filter dyes, wetting agents hardening agents, anti-foggants, selective desensitizers, pH- and pAg-regulating agents, colour couplers, plasticizers, antioxidants and developing agents. On the contrary, the removal of oxygen during the preparation of emulsions containing reducing compounds, such as direct-positive emulsions or emulsions containing a developing agent, is interesting for maintaning the redoxpotential of the light-sensitive composition during its preparation very constant.

In order to avoid foaming during the elimination of gases dissolved in the silver halide emulsion it may be desirable to use an anti-foaming agent.

The apparatus for producing a reduced pressure are sufliciently known and one can choose the proper apparatus for his specific problem.

The following examples illustrate the present invention.

EXAMPLE 1 A fine grain silver bromide emulsion containing 4 mole percent of iodide was prepared by reaction of silver nitrate and an alkali metal halide under atmospheric pressure conditions. The gelatin concentration was 7% by weight and the halide concentration in excess of 10* mole/litre.

The emulsion was divided in three aliquot portions. A first portion was set immediately and washed. A second portion was physically ripened under normal atmospheric conditions at 47 C. for 2 hours. The ripened emulsion was set, shredded and washed. The third portion was deaerated under reduced pressure (65 mm. Hg) for 15 min. Then the said emulsion portion was ripened whilst the reduced pressure was maintained for 2 hours at 47 C. Then the emulsion portion still under reduced pressure was set and shredded but washed under atmospheric conditions.

The determination of the average particle size diameter and standard deviation giving an idea of particle size distribution was carried out by means of an electron microscope and counting machine a described by W. Peelaers, Sci. & Ind. Phot. 2 Series, 38, pp. 33-37 (1967).

A fine grain silver chlorobromide emulsion containing 50 mole percent of bromide was divided into two equal portions. The first portion was converted in a pure silver bromide emulsion by the addition under atmospheric conditions of an amount of potassium bromide equivalent to the silver chloride present in the non-converted emulsion. The total amount of halide in excess after conversion (mainly chlorides) was 0.66 mole per litre. The conversion time was 75 minutes and the temperature 60 C. After the conversion the emulsion was set and washed. The second portion was deaerated for 15 minutes under reduced pressure mm. Hg). Then the conversion was carried out as for the first portion but under a reduced pressure of 45 mm. Hg. The emulsion was set under reduced pressure and washed under atmospheric conditions.

Silver bromide was prepared by adding rapidly 0.1 N silver nitrate solution to 0.1 N potassium bromide solution at 25 C. After 48 h. of sedimentation the silver bromide grains deposited were suspended in water again. The pH of this suspension containing about 50 g. of silver bromide per litre was 6.5 and the pAg 8.5. I

Gelatin was added until a concentration of 5% by weight, and further a potassium bromide excess of 0.15 mole per litre. This emulsion was divided in two equal portions. The first was ripened for 1 hour at C. under atmospheric conditions, whereupon it was set and washed. The second portion was deaerated under reduced pressure (50 mm. Hg) and then ripened for 1 hour at 60 C., set under reduced pressure and washed under atmospheric conditions.

The average particle size in portion 1 and 2 was In and 0.25 respectively.

We claim:

1. In a method of manufacturing fine grain photographc silver halide emulsion materials including the steps of precipitating the silver halide grains, chemcally ripening the resultant emulsion, storing said emulsion prior to coatng and coating said emulsion onto a support, the improvement of carrying out said steps in the absence of ammonia and of carrying out at least one of said steps under a sub-atmospheric pressure not higher than 100 mm. Hg.

2. The method of claim 1, wherein said photographic material has a photographic emulsion that has been prepared under said reduced pressure and includes the additional step of converting the silver halide under said reduced pressure conditions.

3. The method of claim 1, wherein the chemical ripening step is performed at a pressure of about 40 to mm. Hg.

References Cited UNITED STATES PATENTS 1,970,496 8/1934 De Boer -7 3,368,895 2/1968 Matejec 9628 FOREIGN PATENTS 676,745 7/1952 Great Britain 9694 1,008,694 11/1965 Great Britain 9694 I. TRAVIS BROWN, Primary Examiner J. R. HIGHTOWER, Assistant Examiner US. Cl. X.R. 96-114.7 

